Even in the voluntary world of transactions, the attitude, both prevalent and correct, is: What have you done for me lately? Rashi (Breishis 13:3) וילך למסעיו, כשחזר ממצרים לארץ כנען היה הולך ולן באכסניות שלן בהם בהליכתו למצרים למדך דרך ארץ שלא ישנה אדם מאכסניא שלו conveys a weak obligation of “customer loyalty”.
Vote for the lesser evil, vote to avenge the innocent, vote for nobody… whatever. But to go against your own interests?!
(Besides, why would we expect to find good middos in the horrible world of politics? The whole thing is an early auction on stolen goods!)